Add a "pwd" utility to userland.

It's implemented as a separate process. How cute is that.
Tasks now have a current working directory. Spawned tasks inherit their
parent task's working directory.
Currently everyone just uses "/" as there's no way to chdir().

Kernel/Syscall.h

@@ -28,6 +28,7 @@ enum Function {
     PosixMunmap = 0x1996,
     GetDirEntries = 0x1997,
     PosixLstat = 0x1998,
+    PosixGetcwd = 0x1999,
 };
 
 void initialize();